Diffraction gratings and other diffractive optical devices have been developed for many applications. For example, so-called segmented diffraction gratings have been suggested for data routing in optical communication systems. Fiber Bragg gratings (FBGs) have been developed for applications including wavelength selection and routing in optical communications as well as numerous applications as optical sensors and in optical remote sensing.
Some methods of producing segmented diffraction gratings are based on holographic techniques, and FBGs have been made by exposing an optical fiber to an interference pattern produced with optical radiation at wavelengths that produce changes in the refractive index of a fiber. In one method, a mask is provided that is configured to provide a selected interference pattern. Ultraviolet radiation (at wavelengths that are typically between about 150 nm and 400 nm) is directed to the mask. A fiber in which an FBG is to be formed is placed in the interference pattern. The fiber is exposed to the interference pattern for a time period sufficient to produce index of refraction changes of a selected magnitude and in a spatial pattern corresponding to the interference pattern.
While methods for producing FBGs using masks can be simple to implement and have adequate manufacturing throughput, the properties of the resulting FBGs depend on the properties of mask used to produce the interference pattern. Such masks and other diffractive structures can be characterized with two beam interferometric methods in which an optical field produced by light transmitted through the mask is interfered with a reference plane wave. The resulting interference pattern is analyzed to provide phase information about the phase of the transmitted optical field. However, such methods have significant disadvantages. It is generally desirable to measure the transmitted phase front in the near field at distances from the diffractive structure that range from a few micrometers to a few millimeters. Configuring two beam interferometers for measurements at such near field distances is difficult. In addition, two beam interferometric methods generally require ultra-stable environments to eliminate phase noise due to mechanical vibrations or variations in refractive indices experienced by either an optical signal field (i.e., the optical field produced by the diffractive structure under test) or the reference optical field.
In view of these shortcomings, improved methods and apparatus are needed for the characterization of optical devices.